The present invention relates to a surge protector for high speed data networks. In particular, the invention relates to a surge protector apparatus providing a matched impedance to twistedpair electrical transmission media.
Conductive wire interconnections continue to be widely used for high speed computer networks, even though they were once believed to be in jeopardy of imminent obsolescence by fiber optic cables. Because many installations have a legacy investment in computer networks based on conductive wires, and due to other economic and easeofuse considerations, it has been found desirable to develop ways of increasing data transmission speeds on existing conductivebased computer network installations, rather than to replace them. Conductive computer networks, based on twistedpair cable technology, continue to be maintained and installed to meet higher operating bandwidth specifications.
Conductive wirebased computer networks are vulnerable to transients or surges due to, for example, lightning strikes in the vicinity of the network. A nearby lightning strike can induce voltage levels in the conductors constituting the network that are sufficiently high to damage terminal equipment connected to the network. The typical solution to this problem is to connect transient voltage suppression devices between the wires of the network and ground, in order to provide a conduction path for high voltage surges to discharge. Adding such devices, however, places a parasitic reactive load on the conductors. With the increasing bandwidth demands required of computer networks, the ability to conduct high speed data transmission can be impaired by radiative losses and crosstalk which occur whenever the pairs of the transmission cable are connected to a surge suppression device. Such impairments include radiative and coupling losses from the connectors employed to attach the wires to the surge suppression device, losses due to impedance mismatch with the suppression device itself, radiative loss due to the necessity to split the individual pairs for connection with voltage suppressors, as well as the parasitic impedance of the voltage suppressors.
In U.S. Pat. No. 5,706,160 there is shown a proposed surge arrester arrangement in which numerous banks of diodes are connected with conductive traces of a circuit board corresponding to the pairs of a twistedpair based signal transmission system. Additionally, the patent proposes the use of a verticallypositioned shield upon the circuit board in order to reduce crosstalk between conductors of the surge arrester. It would be desirable to provide a surge arrester for high speed computer networks which employed fewer parts and was simpler in overall assembly.
According to one aspect of the invention a surge arrester for twistedpair transmission systems is provided in which each pair of a transmission cable is connected to a circuit board on which an impedancematching stripline is formed on one side of the circuit board. Corresponding isolated ground plane areas are provided on the other side of the circuit board in order to confine radiated signals to the respective vicinities of the striplines. A threshold voltage conduction circuit is connected along each stripline. The connection to the threshold voltage conduction circuit may be geometrically formed as a microstrip to provide a compensating reactance in series with the threshold voltage conduction circuit. Additionally, the threshold conduction circuit may comprise a series combination of threshold voltage conduction devices in order to present a selected capacitive load to the microstrip.